1. Field of the Invention
The present invention relates to a DSL modem apparatus and an initialization method for DSL communication applicable to a multi-carrier method data transmission apparatus that transmits data using a plurality of carriers with various frequencies.
2. Description of Related Art
The ADSL (Asymmetric Digital Subscriber Line) is a service that employs an existing telephone line so that both a high speed Internet connection service and ordinary telephone service can use the same line, which has rapidly become available in the recent years. For providing such ADSL services, ITU-T recommendations have been issued regarding ADSL modems. G.lite and G.dmt recommendations established in 1999 at ITU-T SG15 are ADSL standard recommendations, which further extend to ANNEX.A, ANNEX.B, ANNEX. C, and etc. for initialization establishing methods according to the communication environment of individual country (or region).
FIGS. 4 and 5 illustrate an initialization sequence according to G.lite or G.dmt. In the figures, the left (C) side illustrates an exchange side apparatus (ATU-C), while the right (R) side illustrates a remote terminal apparatus, e.g., at home (ATU-R).
During the initialization sequence, immediately after C-QUIET2 and R-QUIET2, the ATU-C starts hyperframe transmission from “Beginning of Hyperframe”. After the ATU-C starts the hyperframe transmission and upon counting a predetermined number of symbols, the ATU-R matches symbol by finding the beginning of the hyperframe. When the ATU-C transmits C-REVERB1 and C-REVERB2, the ATU-R makes a final adjustment to the symbol synchronization for the reception symbol at location “Last symbol may be shortened by n samples”. The symbol synchronization is performed by forcefully shortening the symbol by n samples, which correspond to a deviation of the symbol synchronization.
As described above, the symbol synchronization of the ATU-R employs C-REVERB1, C-REVERB2, or C-REVERB3 signal received from the ATU-C. This is due to the fact that a REVERB signal includes carriers ranging #32–#127, or #32–#255, and the recommendation clearly states what kind of signal is being sent from the ATU-C, which is useful for establishing a synchronization.
Hereafter, the symbol synchronization method at the ATU-R is illustrated. When the IFFT input signal of C-REVERB at the transmission side (data before calculation, although the transmission side performs the inverse fast Fourier transform on a bit stream that configures a symbol) is X(f), and the FFT input signal of the received signal without the symbol synchronization at the reception side (signal before calculation, although the reception side performs the fast Fourier transform) is Y(f), the following calculation at the reception side can be provided:Z(f)=Y(f)/X(f)When impulse response is res (t), the following calculation is provided:res (t)=inversFFT (Z(f))The above formula calculates the impulse response (res (t)) on the line.
FIG. 6 illustrates a waveform of the impulse response calculated at the reception side. The receiver side locates a peak of the impulse response as illustrated in FIG. 6, and detects the beginning of the C-REVERB symbol. The above-described symbol matching method is an example that can be applied to ANNEX. A specification. Other suitable methods can be applied to different specifications.
When the initialization sequence is completed, data transmission (SHOWTIME) is started. ADSL communication employs the interleave method that is resistant to a burst error. The interleave method sorts data up to a predetermined symbol in a predetermined rule for transmission and restores the reception data by resorting at the reception side. Data sorting is performed within a memory having enough capacity for the interleave process (interleave memory). ADSL communication apparatus sorts data by opening transmission data within the interleave memory at the time of transmission, and resorts data into the original order by opening the reception data within the interleave memory for reception.
An improvement of reducing the number of parts within the apparatus can be made, when the reception timing adjustment for symbol matching during the initialization sequence is designed so that the processor can process as software. This is because the circuit for calculating the reception timing adjustment can be eliminated.
However, when the reception timing adjustment is performed by the processor as software, the production cost may rise because a memory having a large enough capacity needs to be installed for the calculation.